Process fluid level and interface measurements are extremely important in fluid processing installations. Such measurements provide knowledge regarding the amount of fluid remaining in a container or vessel. Additionally, interface measurements can provide important information regarding level(s) at which two or more dissimilar fluids interface. Since level and interface measurements are vitally important to any process installation where knowledge regarding the amount of process fluid remaining is important, such applications are widespread.
Cage guided or bridle mounted level systems are commonly used for level and interface measurement on large process vessels, such as chemical and refinery reactors. A bridle, or cage, is simply a small vessel or conduit that is mounted off of the large vessel and coupled to the large vessel via conduits. The fluid level, and/or interface levels within the cage or bridle are directly representative of such conditions within the larger vessel. However, it is often easier to measure characteristics of the fluids within the cage or bridle. Further, it is known to provide valves to allow the cage or bridle to be fluidically isolated from the larger vessel when maintenance operations need to be performed.
In typical instrument applications, a displacer or “float” is mounted inside the bridle. The buoyancy of the float is translated through the pressure wall via a torque tube and hence into a level transmitter that converts the torque tube force into a pneumatic, analog or digital signal for output. One example of a commercially available digital level transmitter utilizing this approach is sold under the trade designation Type DL3 Digital Level Transmitter, available from Fisher Controls International of Marshalltown, Iowa.
Another type of cage guided or bridle mounted level system utilizes guided wave radar or capacitance-based measurement devices. Typically, the radar waves are guided down through the process liquid in the cage via a waveguide. As the microwave energy reaches an interface, such as the upper level of the process fluid, or an interface between two fluids, a reflection is caused that propagates back along the waveguide to the transmitter. Information about the echo can be used to calculate the level(s) of fluids within the cage. One difficulty encountered by radar-based level measurement techniques is that it is sometimes difficult and/or costly to form a proper seal that, while containing the pressure of the vessel, can still pass the radar signals effectively. For example, it is not uncommon for such vessels to be operating at pressures of several hundred pounds per square inch.